1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting/receiving data in a mobile communication system. More particularly, the present invention relates to an apparatus and method for transmitting/receiving broadcast data in a mobile communication system.
2. Description of the Related Art
In general, mobile communication systems have been developed to support unicast service. The “unicast service” refers to the communication between a base station and one mobile station. That is, in the unicast service, the base station transmits data only to one mobile station, instead of transmitting the same data to a plurality of mobile stations. The voice service and various data services are the typical unicast services.
With the rapid progress of wireless communication technology, systems capable of enabling users to receive broadcast service while on the move have recently been developed and deployment thereof is at hand. The “broadcast service” refers to a process of transmitting the same service data from a base station to a plurality of mobile stations. The broadcast service can provide not only general over-the-air broadcast service but also a plurality of private broadcast services, and deployment of Digital Multimedia Broadcasting (DMB) service is close at hand.
Various attempts are being made to provide broadcast service even in mobile communication systems, and many schemes have been proposed so far. A High Rate Packet Data (HRPD) system proposed in 3rd Generation Partnership Project 2 (3GPP2) is a typical example of the system capable of supporting broadcast service among the mobile communication systems proposed up to now.
The HRPD system adopts a unicast transmission method as its basic transmission method, and also employs hybrid Automatic Repeat Request (H-ARQ). In addition, a Code Division Multiplexing (CDM)-based transmission method and an Orthogonal Frequency Division Multiplexing (OFDM)-based transmission method have been proposed as an example of a transmission method for Broadcast/Multicast Service (BCMCS) recently discussed in the mobile communication system. Herein, “BCMCS” refers to the broadcast service provided in the HRPD system, for convenience. A method for transmitting broadcast signals using H-ARQ uses a scheme for dividing one encoder packet (EP) into a plurality of sub-packets and transmitting the sub-packets using a plurality of slots, like the unicast scheme. A receiver receiving the sub-packets performs decoding thereon by combining the sub-packets using an Incremental Redundancy (IR) scheme. That is, a difference between the Broadcast/Multicast (BCMC) transmission scheme and the unicast transmission scheme lies in that if a mobile station transmits no response signal (ACK/NACK) to a base station in response to received data, the base station transmits a plurality of sub-packets constituting an encoder packet for a predetermined time corresponding to a predetermined number of slots. In the HRPD system, the unicast scheme and the BCMC scheme use the common encoding and decoding methods, and the well-known turbo encoding scheme can be used as the encoding method.
Most mobile communication systems for transmission packets, including the HRPD system, transmit data using multi-slot interlacing schemes. Of the multi-slot interlacing schemes, a 4-slot interlacing scheme is most typical.
With reference to FIG. 1, a description will now be made of the 4-slot interlacing scheme.
FIG. 1 is a timing diagram for a description of a 4-slot interlacing scheme used in an HRPD system.
As illustrated in FIG. 1, a transmitter transmits data to a receiver at intervals of 4 slots. That is, the transmitter performs first transmission (1st TX) 100 at a time of t0 to t1. The transmitted signal is received at the receiver before a time t2, and then processed. Thereafter, the receiver transmits a response signal (ACK/NACK) 105 in response to the first received signal 100. The response signal arrives at the transmitter before a time t4. If the response signal transmitted by the receiver indicates ACK (Good Reception), the transmitter transmits the next data. However, if the response signal transmitted by the receiver indicates NACK (Poor Reception), the transmitter retransmits the first transmitted data. For the retransmission, the transmitter performs second transmission (2nd TX) 110 in response to the response signal from the receiver at a time of t5 to t6. Similarly, the transmitted signal is received at the receiver before a time t7, and then processed. The receiver transmits a response signal (ACK/NACK) 115 in response to the received signal. The response signal arrives at the transmitter before a time t9.
As described above, based on the response signal from the receiver, the transmitter determines whether it will perform initial transmission or retransmission on the transmission data. That is, the transmitter performs the transmission at intervals of 4 slots. Therefore, for the remaining 3 slots where transmission to the receiver is not performed, the transmitter can transmit data to another mobile station, or can transmit data other than the currently transmitted data to the receiver using the remaining slots. This scheme is called the 4-slot interlacing scheme.
The 4-slot interlacing scheme is used for the following reasons. After the transmitter transmits a part of or all of the coded symbol created using one packet, if the receiver fails to receive the transmitted coded symbol, the transmitter should retransmit a part of or all of the coded symbol of the corresponding packet to increase reception capability of the receiver. In the HRPD system, the maximum number of retransmissions is limited to a predetermined value.
The use of the 4-slot interlacing scheme can provide different broadcast services at intervals of a predetermined number of slots. A description thereof will be made with reference to FIG. 2.
FIG. 2 is a timing diagram for a description of a scenario in which different broadcast services are provided at intervals of every slot in an HRPD system supporting a 4-slot interlacing scheme.
It is assumed in FIG. 2 that each of the parts hatched with oblique lines indicates a slot corresponding to a multiple of 4 (4n), and each of the parts hatched with horizontal lines indicates a slot that comes one slot after the multiple of 4. The slots hatched with the oblique lines and the slots hatched with the horizontal lines are the slots allocated for a particular broadcast service. It can be noted that the 4-slot interlacing scheme is applied even to the slots allocated for the broadcast service. A description will now be made of an exemplary method for transmitting packet data through each of the slots allocated for the broadcast service.
In FIG. 2, reference numeral 211 represents initial transmission of a first packet P1, reference numeral 212 represents first retransmission of the first packet P1, and reference numeral 213 represents second retransmission of the first packet P1. Thereafter, in the same slot of the interlacing scheme, the next packet, in other words a third packet P3 231 is transmitted. Although the number of transmissions is set such that one data packet can be transmitted up to 3, the number of transmissions is subject to change. Similarly, for a second packet P2, initial transmission 221, first retransmission 222 and second retransmission 223 are performed, and thereafter, the next packet, in other words a fourth packet P4 241 is transmitted. Similarly, for the third packet P3 and the fourth packet P4, initial transmissions 231 and 241, first retransmissions 232 and 242, and second retransmissions 233 and 243 are performed.
Various messages provided in the system to provide the broadcast service and a method for providing the broadcast service will now be described in detail hereinbelow. The HRPD system transmits packet transmission information for receipt of a broadcast physical layer packet (hereinafter simply referred to as “packet” ) for BCMCS using an overhead signaling message, for example a broadcast overhead message. The overhead signaling message includes therein a BCMC flow ID transmitted in the cell, information on Frequency Allocation (FA) for transmitting each BCMCS packet, position information of transmission slots, a data rate, the number of transmission slots, and Reed-Solomon (RS) coding information. A mobile station, after receiving the BCMCS overhead message, receives a corresponding packet in a corresponding slot using transmission information of the BCMCS packet that the user desires to receive.
Generally, because the BCMCS transmits the same information to a plurality of mobile stations, every base station transmits the same packets in a BCMCS slot in order to provide the BCMCS in the mobile communication system. The mobile station receives the packets from the base stations at once, and increases its reception performance through soft combining, noticeably increasing the performance compared with the method of receiving packets using a signal received from one cell.
The BCMCS transmits data over multiple slots by using the same transmission format in each slot. In this case, the BCMCS increases the number of retransmissions in the area where the reception performance is lower. The BCMCS uses the same transmission format, in other words the same OFDM symbol structure or modulation scheme, even in the area where the number of transmissions slots increases.
A structure of the OFDM symbol used in the BCMCS is designed taking into account the surrounding environment such as the maximum signal delay in the area where the receiver is located. Therefore, at the retransmission time, only some neighbor cells participate in the retransmission, reducing the maximum signal delay value of the OFDM signal that the receiver desires to receive. Generally, a Cyclic Prefix (CP) is inserted in the OFDM symbol taking the maximum signal delay into consideration. A size of the CP depends upon the possible amount of transmission data. That is, an increase in the size of the CP causes a decrease in the possible amount of transmission data, and a decrease in the size of the CP increases the possible amount of transmission data. However, the current BCMCS scheme does not take the changed surrounding environment into account during retransmission, and uses the fixed transmission format. As a result, the CP is set unnecessarily long, causing a waste of radio resources.
Accordingly, there is a need for an improved apparatus and method for transmitting and receiving broadcast data in a mobile communication system.